According to the trends toward the complexity of an electronic apparatus, many wiring lines are substituted with a circuit board. It is known that the circuit board can reduce space, weight, and labor, and is more reliable than the wiring lines. In addition, with the reduction in thickness and light-weight of the electronic apparatus, existing hard printed circuit boards are chiefly substituted with flexible printed circuit boards.
A metallic laminate used for a flexible printed circuit board is roughly divided into a three-layered copper clad laminate (3CCL: 3-Copper-Clad-Laminate) and a two-layered copper clad laminate (2CCL: 2-Copper-Clad-Laminate).
The three-layered copper clad laminate is formed by compressing a copper foil and a polyimide film using an epoxy-based or acryl-based adhesive, and the two-layered copper clad laminate is composed of only polyimide and a copper foil without using an adhesive.
As a method of producing the two-layered copper clad laminate, there are known a sputtering method that deposits and laminates copper on a surface of a polyimide (PI) film and a casting method that coats a polyimide precursor solution on a copper foil, dries the polyimide precursor solution, and thermally or chemically imidizes (cures) the polyimide precursor solution, thereby forming a laminate.
As an example, Japanese Unexamined Patent Publication No. 8-250860 discloses a two-layered copper clad laminate type printed circuit board that is produced using the casting method to have a three-layered laminated polyimide insulating layer, thereby reducing occurrence of a curl.
Here, the three-layered laminated polyimide insulating layer is composed of a first polyimide resin layer that is formed by coating a polyimide precursor solution on a surface of a copper foil as a conductor and imidizing the polyimide precursor solution to have a coefficient of thermal expansion of 20×10−6/K or more, a second polyimide resin layer that is formed on the first polyimide resin layer in the same manner to have a coefficient of thermal expansion of 20×10−6/K or less, and a third polyimide resin layer that is formed on the second polyimide resin layer in the same manner to have a coefficient of thermal expansion of 20×10−6/K or more. In this case, the first polyimide resin layer that is formed on a copper foil layer as an adhesive layer is formed to have a soft physical property, thereby improving adhesiveness to the copper foil.
As shown in FIG. 8, the polyimide resin layer 120 composed of the first to third polyimide resin layers and the copper foil layer formed on the polyimide resin layer 120 are etched to form a copper foil circuit pattern 110a in the copper clad laminate. Then, an IC chip 140 that has an Au bump 141 for signal processing of the IC chip 140 is bonded on the copper clad laminate using a stage 130 and a tool bonder 131.
When the copper foil circuit pattern 110a and the Au bump 141 of the IC chip 140 are bonded, a temperature of the tool bonder 131 is high, for example, at 350 to 450° C. Accordingly, as shown in FIG. 9, there is a problem in that the first polyimide resin layer of the polyimide resin layer 120 that is disposed below the copper foil circuit pattern 110a is softened, and the copper foil circuit pattern 110a is compressed toward the polyimide resin layer 120.
Further, the first polyimide resin layer has the coefficient of thermal expansion of 20×10−6/K or more, which is different from the copper foil. Accordingly, there is a problem in that high-temperature adhesiveness between the copper foil and the first polyimide resin layer of the polyimide resin layer is degraded due to thermal expansion on the above-described high-temperature condition.